My 04 durimax has a straight pipe, no cat or muffler. Had it smogged at jiffy lube next to hot rod hill on American Pacific. They never said anything. It passed fine. Just send the wife to do the dirty work, if they say anything she can say I don't know what you are talking about, I'll have to talk w/ the hubby. Good luck with whatever you do.
Oops, I guess cat is still there just hollowed out. I don't know if any of this helps now, but good luck again.